The present invention relates to a power saving device for use in a power supply circuit of the type using a DC-DC converter.
Power supply circuits of the type in which a voltage supplied from a low-voltage power source is stepped up by a DC-DC converter and supplied to a load have been widely used in various electric equipment and especially in small-sized equipment. In general, these power supply circuits are actuated by closing a mechanical start switch such as a slide switch. Therefore, in the case of, for instance, an electronic flash device, it occurs very often that one fails to open the start switch before the electronic flash device is inserted back into its case or put into a pocket. Moreover, even when the start switch is opened, it is very often closed erratically again when it is inserted into the case or put into the pocket. As a result, the power source is wasted for nothing.
In order to overcome this problem, there have been proposed various types of automatic power-supply shutoff devices. In one type, a mechanical switch and the so-called "off" type magnet, which is energized when supplied with a current, are combined with a timer so that after a predetermined time interval, which is set by the timer, the power supply is automatically shut off. More specifically, an electromagnet is operatively combined with a mechanical switch which is normally opened to shut off the supply of power to a DC-DC converter. The electromagnet is energized to force the normally open switch to be closed against its biasing force so that the supply of power to the DC-DC converter can be maintained only for a predetermined time interval. After a predetermined time interval, the electromagnet is de-energized so that the mechanical normally open switch is forced to open by the biasing force. To put into another way, when the magnet is energized the on-off switch is closed so that the power is supplied from the power source to the load. However, depending upon the load, when the magnet is energized, a large current flows into the DC-DC converter from the power source so that the voltage across the power source drops exceedingly to such an extent that the electromagnet cannot be maintained energized. As a consequence, the power supply is interrupted and the electric equipment cannot be activated.
Moreover, there arises another problem. That is, the automatic power-supply shutoff device of the type includes the electromagnet and its control system so that it becomes large in size and complex in construction. Therefore, it is not adapted to be incorporated into small-size electric equipment.
There has been also devised and demonstrated another automatic power-supply shutoff device of the type utilizing a mechanical switch and the so-called "ON" type magnet which is a permanent magnet so designed and arranged as to be demagnetized when the coil mounted thereon is energized. The demagnetization time interval is controlled by a timer circuit. In contrast with the automatic shutoff device of the type utilizing the "off" type magnet, the mechanical switch can be maintained in a desired state by the attractive force of the permanent magnet. When the switch is closed, it can be maintained in the closed state in a stable manner even when a large current is supplied from the power source to the load. However, this type of device also has a drawback. That is, a predetermined time interval which is set by the timer circuit, the demagnetizing current is instantaneously supplied to the demagnetizing coil so that the switch is returned to the open state. Therefore, the permanent magnet is demagnetized many times, its attractive force is so diminished that it cannot hold the switch in the closed state. Moreover, when the power source is used many times, the current cannot be supplied which is sufficient enough to demagnetize the permanent magnet holding the switch and consequently the power is kept supplied. In addition, the permanent magnet used is large in size and expensive. If a poor quality permanent magnet is used for the sake of savings in cost, reliability in operation would be inevitably degraded.
As described above, the prior art power saving devices have various problems as described above and the present invention was made to overcome them.